Cell-culture methods are being used to investigate the regulation of growth and differentiation of mammary epithelial cells of the mouse. Normal, preneoplastic, and neoplastic (adenocarcinoma) cells are all being grown in primary monolayer culture. We have recently discovered that these cultures are not really monolayers, since viable dividing cells spontaneously detach and float into the supporting culture fluid. This phenomenon bears a notable resemblance to sloughing of cells from mammary tumors in vivo. Therefore, we are investigating why the cells detach from monolayers. Adrenal cortical hormones prevent this release; other hormones have small effects, if any. We are analyzing cell-surface composition of cells that detach and are comparing it with surface composition of non-detached cells. Growth properties, including cell kinetics of adherent and of released cells, are being determined and compared. In conjunction with these comparative studies of growth regulation, we are investigating controls on mammary differentiation in monolayer cultures. We have devised a sensitive method for detecting newly-made lactose in culture fluids, and have succeeded in stimulating lactose synthesis by normal mammary epithelium. We are now commencing efforts to coerce preneoplastic and neoplastic cells to differentiate and make lactose. If we cannot do so, we will attempt to find the lesion in cell constitution which prevents these cells from differentiating normally.